1. Field of the Invention
The present invention relates to a heat exchanger block.
German Utility Model DE 296 04 521 U1 discloses a heat exchanger body composed of plates made of graphite. Passage systems for two media are disposed inside the heat exchanger body.
The passages for the gaseous medium giving off heat (referred to below as flue-gas passages) are formed by grooves which are incorporated in abutting surfaces of the plates and between which ribs remain. At least two plates of that kind are combined with one another in such a way that the grooves in the abutting surfaces of the two plates complement one another and in that way form passages which are defined by the abutting ribs of both plates.
The passages for the second medium to be heated (referred to below as cooling medium) are constructed as bores passing through the plates. The thickness of the plates is selected in such a way that only a thin material barrier which does not impair the heat transfer to a great extent is located between the two passage systems. However, the thickness of that material barrier is sufficient to separate the passage systems from one another in a fluid-tight manner and ensure mechanical stability.
Those surfaces of the plates which face outwards are flat. The plates are held together by adhesive or through the use of seals and tie rods. Several pairs of plates can be placed side by side or against one another. That modular type of construction permits specific adaptation of the capacity of the heat exchanger for various requirements.
The passage systems may be disposed parallel to one another or perpendicularly to one another, depending on whether it is intended to direct the media in counter-flow or co-current flow or in cross-flow. However, a considerably higher construction and processing cost is required for directing the media in parallel, in order to achieve the separation of the flow to be cooled and the flow to be heated.
If the media are directed in parallel, the orifices of both passage systems lie on the same end faces of the plates. That is to say, respective connection systems (head pieces) are to be provided for two separate media flows at the relevant end faces.
In order to keep the material barrier between the two passage systems as small as possible, the bores run between the grooves incorporated in the plate surfaces, i.e. they lie on a plane close to or above the bottoms of the grooves. The passage systems are, as it were, interlaced. The result thereof is that the orifices of the flue-gas passages and the orifices of the cooling passages lie very close together at the end faces of the plates. Specially constructed head pieces are therefore necessary for feeding and distributing the media to the respective passage system or for collecting the partial flows from the passages and for removing the media. The head pieces enable different media to be supplied and removed separately in the narrowest space.
As an alternative, it is proposed in German Utility Model DE 296 04 521 U1 to close the ends of the bores at the end faces, for example by plugs adhesively bonded in place, and to provide branch bores from the plate surfaces to the bores forming the cooling passages, so that the supply and removal of the cooling medium can be effected from the plate surface. Although that variant solves the problem of space at the end faces, it is even more complicated in production, since the end-face orifices have to be closed in a fluid-tight manner at each bore and two branch bores must additionally be provided.
Directing the media in cross-flow is therefore preferred in practice, although more effective cooling can be achieved by directing the media in counter-flow.
The flue-gas passages are preferably constructed in such a way that firstly a high ratio of heat transfer area (wall area) to passage volume is achieved and secondly the cross section of flow is sufficient in order to ensure the outflow of the gases by natural convection. This is achieved by passages in the form of slots having a high ratio of depth to width. The grooves forming the flue-gas passages are produced mainly by milling.
The passages for the cooling medium always have a circular cross section, since they are bored. However, the construction of those passages as bores is disadvantageous due to the high processing efforts.
In addition, the limitation to circular passage cross sections due to the boring operation is unfavorable for the heat transfer. If the form of the passages is fixed, the heat transfer coefficient alpha between wall area and cooling medium, which in turns depends, inter alia, on the flow state of the cooling medium and on the geometrical shape of the heat transfer area, can only be increased by increasing the flow velocity of the cooling medium in the bores.